Television synchronization



April 23, 1935. J. v. L. HOGAN- TELEVISION SYNCHRONIZATION Filed July 1, 1930 QDDDDEDDDUDDM /%WENTOR A ORNEY Patented Apr. 23, 1935 PATENT OFFICE TELEVISION SYNCHRONIZATION John V. L. Hogan, Forest Hills, N. Y., minor to Radio Inventions, Inc., New York, N. Y., a corporation of New York Application July 1, 1930, Serial No. 465,126

4 Claims.

The present invention relates to a method and means for synchronization of television transmitters and receivers. It relates particularly to systems of synchronization where a single channel of communication is employed for the transmission of the image signals and the synchronization signals.

In systems of this type as at present employed,

transmission of the image signals is momentarily suspended to allow the transmission of synchronizing signals. These latter signals are of the same general nature as signals produced by the closure of a circuit where the full electromotive force is thereby impressed upon the line, resulting in the propagation of a signal possessed of an extremely steep wave front form, of the general type often referred to as square. Television systems in general are so designed that wave forms will be transmitted therethrough with the minimum amount of distortion. This causes the arrival at the receiving end of the system of a wave form for synchronization purposes which in general remains of this square type.

Now wave forms of this steep front type are likewise frequently produced by extraneous impulses which infiuence transmission systems, especially radio transmission systems. This brings about the ready confusion of such stray signals with the regular signals sent for the purposes of synchronization. Moreover, such impulses have their energy distributed over a wide range of frequencies and do not lend themselves effectively to resonant selection or tuning.

One purpose of my invention is to send synchronizing signals which shall be of such nature as to be readily distinguished from the abovementioned stray signals.

Another object of my invention is to send for purposes of synchronization signals of such inherent characteristics that advantage may be taken of the properties of syntonic or resonant circuits at the receiving end of the system, not only to make such ready distinguishment of synchronizing from stray signals, but also to facilitate the-building up of tuned impulses by the action of the incoming synchronizing signals, with respect to the electrical constants of these syntonic circuits.

A further object of this invention is. to cause a synchronizing signal of a given intensity to make available at the receiver a greater amount of energy than would otherwise be available. It is well known in the art that when a certain signal is to be amplified, the ratio of signal to stray energy in the output of the amplifier can be made to assume a much more favorable value when the desired signal is of a certain wave form having properties commonly expressed by the trigonometrical term sine, than where it par- 5 takes of any other form. The commonly used wave form with rectangular characteristics, is far inferior in this respect to the sine form, which is chosen as the preferred one in the employment of my invention. 10

My invention broadly considered comprises an apparatus and process for the production of a substantially sinusoidal wave by suitably modulating the optical input of a. photo-electric cell, then selecting this wave (preferably by appro priately tuned circuits) at the receiver, and utilizing the same for purposes of synchronization.

By the use of the word sinusoidal in the specification and claims I refer to signals whose wave front is substantially less steep than 90 and which approach the general character of a true' sine wave.

Other purposes'of my invention will be apparent from the following description and drawing, where Fig. 1 illustrates certain essential features of a television transmitter embodying my invention in connection with its scanning mechanism.

Fig. 2 illustrates graphically the synchronizing and signal wave typical of a transmitter, not employing my invention and scanning the film of Fig. 1 at the point MN.

Fig. 3 graphically shows the form of wave typical of my invention.

Fig. 4 shows an alternative form of mask for the production of synchronizing signals.

Fig. 5 illustrates the wave form produced by this alternative form of mask.

Fig. 6 schematically shows one type of receiver for utilizing my invention.

Fig. 7 shows a section of film having one form of the mask of my invention enregistered thereupon.

Referring to Fig. 1, a scanning member, for instance a disc ID, with slits H, is in rotation past a mask l2. The slits ll upon the disc cooperate with a fixed slit H in a screen M, to scan film I, carrying images 5, which film ,is moving vertically between these scanning members and a light source (not shown).

A light sensitive cell, likewise not shown, is situate the other side of the scanning device and is so positioned and shielded as to be influenced by the optical output of the scanner in the customary fashion well known in the art. An ad justable unitary (as shown) or divided mask 2 is fastened by thumb screws l3 to supports behind it and not visible in the figure. Slots l6 allow the lateral adjustment of this mask or its parts. The mask is so positioned as to cause periodical intervals of darkness to ensue upon the rotation of the scanning disc, substantially as set forth in my copending application, but in addition has its opacity on one or both sides varying in degree, or in other words varying from virtually complete transparency at one end to substantial opacity at the other end of the lightactive portions thereof. If this gradation of light-passing ability be properly designed, so that its effective action upon the light-sensitive cell varies according to the sine law, the electrical output of 'the cell which is used for synchronizing purposes will be substantially of the same sine form, as indicated at AC and DF of Fig. 3.

Suitable motivating devices, sources of mechanical and electrical energy, amplifiers, and other auxiliary apparatus as commonly employed in television transmitters, are employed in conjunction with the apparatus shown, to complete the transmitter. The letters A, C, D, F indicate distances corresponding to those indicated upon Fig. 2.

Fig. 2 shows the character of wave form which the device of Fig. l scanning film 5 at the horizontal section line M-N would produce, were there not used the mask of the special shaded type shown therein. It is to be noted that this wave form, derived from the use of a mask which causes the sudden extinguishment of the light impulses, indicates the synchronizing signals AC and D'F as shown in this figure to be of a wave form of the square type previously mentioned.

Fig. 3 indicates the picture signals C D separated by synchronizing signals AC and D'F, which latter are of approximately sinusoidal form. It is also desirable that the interval CD' be 3, '7, 11, 15 times the interval AC. This allows the successive synchronizing signals to be in proper phase relationship, so that each will reinforce any current which may still be persistent in the synchronizing circuits of the receiver. In the extreme case, the oscillation of the output circuits of the synchronizing amplifiers at the receiver, as shown in a subsequent figure, may be maintained substantially continuously by embodying therewith well known oscillatory circuits, so that the signals received act as a corrective timing factor. In this case the relationship of synchronizing to picture "signals just mentioned is particularly desirable.

Fig. 4 shows an alternative form of mask for use according to my invention. This mask carries a shading pattern producing a. greater portion of a complete cycle, so that the signals produced thereby shall be of a higher frequency, and in addition, the resonance effect he more pronounced due to the more complete character of the signal wave produced thereby.

Fig. 5 shows one type of wave form produced by this further use of the principle of the production of sine wave forms by the use of a shaded mask. By making the shading of this mask of a multiple character, that is making it pass from light to darkness and then return again to light, a complete half cycle of the wave form can be produced. This effect can be reduplicated so that a greater number of half or complete waves are produced, if desired, of any frequency within practical limits. The more nearly complete the wave form produced, or the greater the number of half cycles employed, the easier the filtration of these signals at the receiver, other considerations being equal.

In this case the interval CD should be an odd multiple of the interval AC, for best results, although not so shown in Fig. 5.

This multiple wave form referred to in the preceding paragraph can be produced by causing the space A-C to contain shading which undergoes within this space the changes indicated in Fig. 4, twice, thus giving rise to a wave form covering one whole cycle.

It is also possible to have at least one mask bear a shading pattern consisting of alternate transparent and opaque portions tapered in shading or not, so disposed as to give a multiple character to the synchronizing signal, which may be of a fiat top nature, of a substantially sinusoidal form, or of other forms best adapted to the immediate purpose.

Fig. 6 shows one form of receiver adapted to utilize the special signals coming from the transmitters just described. 3| and 34 are the extremities of an ordinary collector system, connected by wires 32 and 33 with a suitable initial tuning and amplifying system 35, powered by an energy supply 23, through conductors 24 and 25.

This system will deliver a demodulated output carrying the picture signals, alternating with the synchronizing signals (preferably of sinusoidal wave form) produced by the transmitters above described. This output is delivered to light reproducing cell 40, and electromagnetic coupling coil 38, by wires 36, 31 and 39. Equivalent systems, either series or parallel connected, and using resistive, capacitative or inductive coupling can be alternatively employed. Coil4| and adjustable capacity 42 form a resonant circuit tuned to be responsive to the synchronizing signals in coil 38. This circuit virtually separates the synchronizing signals from the picture signals. Conductors 43, 44 lead the output of this filter system to amplifier system 45, powered by conductors 21 and 28 from source 26, and incorporating within itself further selective or filter systems, if so desired.

In this same unit can be incorporated regenerative systems of well known type, such as those employing thermionic vacuum tubes of polyiodic type. These systems can be adjusted to reduce the damping of the circuits, and by proper adjustment to give an output in phase with the received synchronizing signals. The condition of incipient or self-oscillation makes it possible for the output to persist after the exciting impulses have ceased, and therefore to be substantially or actually continuous, in which case the structure becomes substantially a generator distantly controlled as to frequency.

Its output is delivered by conductors 46, 41 to suitable electromagnetic structures 49, interconnected as shown by wire 48.

These last devices operate upon magnetically responsive masses such as an iron disk 20 having circumferentially disposed rectangular holes 20. This disc 20' revolves upon the same shaft 2| which conveys the energy of rotation of the motor 29 to the scanning rotary member 22. The narrow strips between the holes 29, indicated by 2|, are attracted by electromagnets 49 when the latter are energized. The design of motor 29 is such that the speed of this shaft 2| can, through such attractions of magnets for strips 2|, undergo alteration and be accelerated or retarded to a degree sufiicient to secure the presence of an armature piece opposite a magnet at the end of each line scanning, resulting in the requisite synchronism without harmfully affecting its relationship to the motor or the current drawn by the latter. Furthermore, the natural speed of this shaft, when the synchronizing device is not in use should be.

as close as feasible to synchronous speed, in order that the energy demanded of the reactive electromagnetic members 49 and 2| may be kept a minimum. In an alternative form the magnetic regulator 492l' may be omitted and the motor 29 constructed of the synchronous type and designed to run on current of the synchronizing signal frequency as supplied from the amplifier 45.

Appropriate screening and optical units, not shown, are used in cooperation with the receiving scanner, in well known fashion, to give the ultimate output of the device such shape, size and position as to be most suitable for observation.

The magnetically responsive members 2| are so positioned in respect to the apertures of the scanning member 22 and to the magnetic control members 49 that the interaction of 49 and 2| will tend to draw the scanning member, which can be a disc, or other shape of scanner, having one or more moving parts, of the several types known in the art, into the proper phase relationship with the position of the scanner at the transmitter corresponding to the production of the synchronizing signals at that moment. These repeated magnetic impulses constitute a means of maintaining the synchrony of the receiver scanner member as it approaches that condition.

The initial synchronizing signals have been shown in the foregoing as being of a negative character. This is not essential and their inversion by electrical means to a positive character is readily accomplished, as disclosed in my copending application Serial No. 457,217. Apparatus is well known in the art whereby electrical impuses can be inverted or have their phase relationship otherwise altered and such apparatus may be used at either transmitter or receiver, or both.

In the foregoing description and figures the length of the synchronizing impulses relative to the length of the picture impulses has for the purposes of clear illustration, been exaggerated over that usually needed. When such a length of impulse is employed, and if the perforations in the film guide track produce disturbing impulses, non-perforated film may be used to avoid modulation of the signals by the holes therein. Likewise it is preferable that the degree of opacity be of a continuous nature rather than of the dotted line type used in the drawing to indicate the degree of opacity.

Fig. 7 shows one method whereby the sinusoidal variations of screen transparency factor, required for the preferred form of my invention, can also be secured from continuous lines 60 disposed longitudinally of the film l' itself and photographically or otherwise enregistered upon the edge of the film. Such continuous lines, shaded or not, may obviate the use of the exterior masks, or may be used in cooperation therewith. They may be duplicated as shown or merely on one edge and may be entirely within the perforated border or superimposed upon it. Furthermore the masks may be enregistered upon the borders of the film at the sides of the perforations and a stationary mask used at the point where the perforations occur in the film m .rgin, thus combining stationary and moving masks.

Sound or silent film may be employed with equal facility with one of these systems, by using such adjustments-and modifications thereof as are disclosed in my copending application Serial No. 457,217. In the masks herein illustrated, the distances of all the coacting parts have been so arranged that the mask carries shading on one side only. If film with sound track be employed, it is preferable that this shaded mask be used on the side opposite the sound track. It is possible by duplication or re-apportionment of the various dimensions shown, or a modification of the same, to utilize shading on both sides.

Wave forms other than of the sine type may be. employed within the broad scope of my invention.

Instead of the use of a single mask structure laterally limiting the active aperture fortransmission of the optical image, it is possible to employ two independent masks, either or both of which may be shaded, one on each side of the opening, since no framing in the vertical direction is needed. The use of these two independent masks allows greater flexibility in the use of my invention with film which may differ from the sizes usually employed. I

It is evident that the level of the synchronizing signal with respect to the level of the image signal will vary according to the nature of the image but will never be less, since the darkest photographic film allows the transmission of some light, while the masks of my invention may be made completely opaque at their maximum density.

Drums, ribbons, and the like, can be substi tuted for the scanning disc herein shown, without altering the principle involved.

Separate sound records may be employed with the use of my invention, substantially as indicated in my copending application.

The electromagnets in my receiving structure have been shown with rectangular cross-section,

but it is possible to employ a cross-section of any suitable form, although a core as shown in the shape of a narrow strip radially disposed relative to disc 20' is one preferred form. This limits the angular distance through which the magnetic forces act most strongly, and thus promotes increased accuracy of synchronization.

My system possesses many advantages of which some are here set forth.

Stray impulses, which are so commonly met with upon transmission channels, especially radio channels, will, as a rule, cause less disturbance with systems utilizing resonant circuits attuned to give maximum response to certain frequencies only.

The use of a certain definite and curved shape of wave for synchronization purposes makes it less likely that a similar signal will be fortuitously produced by scanning of the optical image, thus giving rise to a false synchronization impulse.

A device at .the receiver made responsive to a steep front wave will inherently have a much broader range of response to wave forms of varying type which may accidentally arise by the scanning of the optical image. Therefore, better discrimination between the synchronization and the television signals will be possible by the use of the wave form employed in my invention.

By the use of suificient regeneration in the synchronizing amplifier at the receiver, the amount of transmitted energy required to effect this synchrony is reduced to a minimum.

By the supply of sufficient energy from the synchronizing amplifier, the size of the motor necessary to rotate the scanning disc can be reduced to a minimum. In case a synchronous type of motor, drawing energy from power lines also feeding the transmitter, is employed, any sudden change of frequency which may occur on the power lines will tend to aiiect the motors at both transmitter and receiver at the same rate of change, even though their natural inertia, percentage load, or other factors afiecting their speed might differ widely.

I claim:

1. In electro-optical synchronizing signal producing apparatus, optical image projection means, light responsive means, interposed means for producing picture signals and interposed light controlling means, including screening members of substantially tapering transparency and adjustable with respect to one another and to the other co-operating parts of said electro-optical synchronizing signal system so as to produce substantially sinusoidal synchronizing signals alternating with other signals, each representing the scanning of a single picture line.

2. The method of automatically synchronizing a television system which includes the steps of alternately transmitting over the same channel synchronizing impulses and groups of picture impulses, and making the time interval during which a single group of picture impulses are transmitted substantially an exact odd multiple of the time interval during which synchronizing impulses are transmitted.

3. A television system including means for producing groups 01' picture signals, each group representing the scanning of a single line, means for producing synchronizing impulses between successive groups 01' picture impulses, means for determining the relative length of the picture impulses and the synchronizing impulses so that the time interval during which a single group of picture impulses are transmitted will be substantially an exact odd multiple of the time interval during which synchronizing impulses are transmitted, and means for alternately transmitting both types of impulses over a single channel.

4. A television system including means for producing groups of picture signals, each group representing the scanning of a single line, means for producing synchronizing impulses between successive groups or picture impulses, means for determining the relative length of the picture impulses and the synchronizing impulses so that successive groups of synchronizing impulses occur at such time intervals that they will act in phase upon a circuit tuned to their frequency, and means for alternately transmitting both types of impulses over a single channel.

JOHN V. L. HOGAN. 

